74 L.P. Davila et al.2.1 Silica Polymorphs
The name quartz comes from the German word “quarz,” of uncertain origin. Quartz
and the other main polymorphs of silica are related in the phase diagram [4] shown
in Fig. 3. Under ambient conditions, α-quartz is the thermodynamically favored
polymorph of silica. At 573°C, α-quartz is transformed into β-quartz, generally
similar in structure but with less distortion. This thermal transformation preserves
the optical activity of quartz. Heating quartz to 867°C leads to the transformation of
β-quartz into β-tridymite, involving the breaking of Si−O bonds to allow the oxygen
tetrahedra to rearrange themselves into a simpler, more open hexagonal structure of
lower density. The quartz–tridymite transformation involves a high activation
energy process that results in loss of the optical activity of quartz. Heating of
β-tridymite to 1,470°C gives β-cristobalite that resembles the structure of diamond
with silicon atoms in the diamond carbon positions and an oxygen atom midway
between each pair of silicon atoms. Further heating of cristobalite results in melting
at 1,723°C. A silica melt is easily transformed into vitreous silica by slow cooling,
resulting in a loss of long-range order but retaining the short-range order of the
silica tetrahedron.
In the last ten years, at least a dozen polymorphs of pure SiO 2 have been reported [6].
Stishovite, another form of silica obtained at high temperatures and pressures, has,
rather than a tetrahedral-based geometry, a rutile (TiO 2 ) structure in which each Si
atom is bonded to six O atoms and each O atom bridges three Si atoms [6]. Stishovite
(found in Meteor Crater, Arizona) is more dense and chemically more inert than nor-
mal silica but reverts to amorphous silica upon heating.
The distinction among polymorphs other than stishovite arises from the different
arrangements of connected tetrahedra. Important examples are quartz and cristobalite.
The structures of these polymorphs are relatively complicated. These structures are
also relatively open, as corner sharing of oxide tetrahedra prevents the close-packing
of anion layers as found in the fcc- and hcp-based oxides [5]. One consequence is that
these crystalline structures have low densities, e.g., quartz has a density of 2.65 g cm−3.
This low density facilitates structural changes and phase transitions at high pressures.
Finally, the high strength of the Si−O interatomic bond corresponds to the relatively
high melting temperature of 1,723°C.
When crystalline silica is melted and then cooled, a disordered 3-dimensional
network of silica tetrahedra (vitreous silica) is generally formed. Glass manufacturing
in the USA is a 10 billion dollar per year industry. It directly benefits from studies of
quartz as one of the main raw materials of commercial glasses is almost pure quartz sand,
with other raw materials being primarily soda ash (Na 2 CO 3 ) and calcite (CaCO 3 ) [1].
2.2 Quartz
Low (α) quartz allows little ionic substitution into its structure. High (β) quartz allows
the charge-balanced substitution of framework silicon by aluminum, with a small
cation (Li+) occupying the interstices. In the more open cristobalite and tridymite
structures, this charge-balanced substitution can be extensive, with many alkali and
alkaline earth ions able to occupy interstitial sites. Such materials are called “stuffed